1. Field of the Invention
The field of the invention relates to shielded cables and methods for manufacturing such cables.
2. Brief Description of the Prior Art
Protective layers or sheaths have been applied to cables for insulating them from the elements, for safety reasons, and other purposes. Three general types of insulation shields are known to the art, particularly in connection with medium voltage cable (5,000-35,000 volts). The first is an extruded shield wherein a polymer is melted, applied over the insulation through a set of dies, and vulcanized. A second type is a fabric tape coated with semiconducting material. The tape is helically applied over the insulation. A third type is a paint tape wherein a semiconducting paint is applied over the insulation to a thickness of about 0.002 inches. A semiconducting tape is applied thereover to prevent the paint from wearing off.
Each of the above described shields are characterized by certain drawbacks. Extrusion requires costly equipment and problems frequently occur in cable terminations when removing the extruded material. The material may either stick too much or too little to the insulation thereby creating problems in terminating or causing corona if adhesion is slight. This is due to the fact that it is often difficult to control the bond. Taped shields are not entirely successful as corona occurs due to the lack of adhesion and the presence of gaps at the overlaps. If paint is employed, extra work is required in its removal.
Various patents are directed to electrical cables having protective layers. U.S. Pat. No. 2,941,911, for example, discloses a tape which is applied and then heated to form a continuous sheath. U.S. Pat. No. 3,790,694 concerns an extruded thermoplastic sheath applied to a cable. U.S. Pat. No. 3,539,409 discloses a process wherein a conductor is first wrapped with a layer of epoxy-impregnated tape. A heat-shrinkable material is wrapped about the tape and heat is applied. During the curing phase, the heat-shrinkable material contracts resulting in external pressure on the epoxy layer. The epoxy thereby flows into any voids which existed prior to curing. Other prior art patents include U.S. Pat. Nos. 3,019,285, 3,422,215, and 4,051,324.